| Adsorption has been used to remove trace organic pollutants in source water inorder to reach the growing higher standards of water quality. Hexagonal mesoporoussilica (HMS) is an emerging adsorbent, which is of low synthesis cost, resistant tohydrothermal conditions, and is of good adsorption selectivity. However, itsregeneration is of much cost and time consuming which hinders its application. Hence,it is important to find a way of regeneration that is in-situ, fast and convenientlyeffective. Ultrasonic treatment is an environmentally friendly technique which has beenapplied and studied in the field of water treatment. The study on the regeneration ofHMS using ultrasound could direct the removal of trace organic pollutants in sourcewater theoretically. Meantime, the application of ultrasound-ozonation could result ingrowing removal rates of trace organic pollutants. Moreover, the joint technique ofultrasound-ozonation would inhence the regeneration of HMS.In this paper, HMS was synthesized under highly acid conditions;Para-chloronitrobenzene (p-CNB) was the target pollutant to be removed by adsorption.The mechanisms of p-CNBs adsorption from water onto HMS and ultrasonic desorptionof p-CNBs from HMS were both studied and analysed. This paper also found theappropriate conditions for the regeneration of HMS by ultrasound-ozonation.HMS synthesized in this paper was a kind of hexagonal mesoporous body whoseparticles look like tiny pies well-distributed. The size of these tiny pies was ofnano-scale. The adsorbent was of great surface areas and there were a few surfacehydroxyl groups in the pore wall. The time of adsorption, the initial concentrations ofp-CNBs, the dose of HMS, water temperature and the pH all affected the adsorptionremoval in a profound way. The removal of p-CNBs by HMS was believed to bemonolayer adsorption and the process was non-reversible. The adsorption was assumedto happen via electrostatic, van der Waals interactions and the strong hydrophobicity ofp-CNBs.In the process of ultrasonic treatment, p-CNBs were removed by mechanicaldesorption from HMS and degraded by direct thermal decomposition (pyrolyzed); andthey also reacted with free radicals (2OH). All these reactions resulted in theregeneration of the absorbent. Experimental results showed that higher sound intensity, longer contact time, higher temperature, and acid pH were optimal for the regeneration ofHMS. The regeneration of HMS was not good after repeated ultrasonic treatments.It was found through SEM (scanning electron microscope), IR (infraredspectroscopy), and nitrogen adsorption measurements that the particles of HMS tendedto get together and began to fuse under ultrasonic compression. Meantime, the surfaceareas of HMS declined while the pore sizes growed with the disappearance of doublepore sizes distribution. It was also revealed that ultrasound would not destroy thestructure groups whereas it would degrade the polymerization of Si-O groups. With thegrowth of sound intensity and contact time, particles of the absorbent gradually fused;and the adsorption capacity dropped grossly under excessive sound intensity.Ozonation alone was effective in degrading p-CNBs in aqueous solution but theregeneration of HMS was not promoted at the same time. The combined processinvolving ultrasound and ozone was then used in the experiments which realved thatlow sound intensity, high concentration of dissolved ozone, short contact time and acidpH were optimal for the generation of HMS. Results showed that the optimumconditions were as followed: I=2.6W/cm~2,[O3]0=4.0mg/L, t=5min, and pH=4.0±0.1(T=25℃). In the combined process, the optimum sound intensity and contact timewere both reduced to1/4of that in ultrasonic process alone. Moreover, The regenerationof HMS was good after repeated ultrasonic-ozonation treatments.. |
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